In lithography for device manufacture, there is an ongoing desire to reduce the size of features in a lithographic pattern in order to increase the density of features on a given substrate area. Patterns of smaller features having critical dimensions (CD) at nano-scale allow for greater concentrations of device or circuit structures, yielding potential improvements in size reduction and manufacturing costs for electronic and other devices. In projection photolithography, the push for smaller features has resulted in the development of technologies such as immersion lithography and extreme ultraviolet (EUV) lithography.
As an alternative, so-called imprint lithography generally involves the use of a “stamp” (often referred to as an imprint template) to transfer a pattern onto a substrate. An advantage of imprint lithography is that the resolution of the features is not limited by, for example, the emission wavelength of a radiation source or the numerical aperture of a projection system. Instead, the resolution is mainly limited to the pattern density on the imprint template.
For both projection photolithography and for imprint lithography, it is desirable to provide high resolution patterning of surfaces, for example of an imprint template or of other substrates. The use of self-assembly of a block copolymers (BCP) has been considered as a potential method for increasing the feature resolution to a smaller dimension than that obtainable by prior lithography methods or as an alternative to electron beam lithography for preparation of imprint templates.
A self-assemblable BCP is a compound useful in nanofabrication because it may undergo an order-disorder transition on cooling below a certain temperature (order-disorder transition temperature To/d) resulting in phase separation of copolymer blocks of different chemical nature to form ordered, chemically distinct domains with dimensions of tens of nanometers or even less than 10 nm. The size and shape of the domains may be controlled by manipulating the molecular weight and composition of the different block types of the copolymer. The interfaces between the domains may have a line width roughness of the order of about 1-5 nm and may be manipulated by modification of the chemical compositions of the blocks of the copolymer.
The feasibility of using a thin film of BCP as a self-assembling template was demonstrated by Chaikin and Register, et al., Science 276, 1401 (1997). Dense arrays of dots and holes with dimensions of 20 nm were transferred from a thin film of poly(styrene-block-isoprene) to a silicon nitride substrate.
A BCP comprises different blocks, each typically comprising one or more identical monomers, and arranged side-by side along the polymer chain. Each block may contain many monomers of its respective type. So, for instance, an A-B BCP may have a plurality of type A monomers in the (or each) A block and a plurality of type B monomers in the (or each) B block. An example of a suitable BCP is, for instance, a polymer having covalently linked blocks of polystyrene (PS) monomer (hydrophobic block) and polymethylmethacrylate (PMMA) monomer (hydrophilic block). Other BCPs with blocks of differing hydrophobicity/hydrophilicity may be useful. For instance a tri-block copolymer such as (A-B-C) BCP may be useful, as may an alternating or periodic BCP e.g. [-A-B-A-B-A-B-]n or [-A-B-C-A-B-C]m where n and m are integers. The blocks may be connected to each other by covalent links in a linear or branched fashion (e.g., a star or branched configuration).
A BCP may form many different phases upon self-assembly, dependent upon the volume fractions of the blocks, degree of polymerization within each block type (i.e. number of monomers of each respective type within each respective block), the optional use of a solvent and surface interactions. When applied in a thin film, geometric confinement may pose additional boundary conditions that may limit the phases formed. In general spherical (e.g. cubic), cylindrical (e.g. tetragonal or hexagonal) and lamellar phases (i.e. self-assembled phases with cubic, hexagonal or lamellar space-filling symmetry) are practically observed in thin films of self-assembled BCPs.
The phase type observed may depend upon the relative molecular volume fractions of the different polymer blocks. For instance, a molecular volume ratio of 80:20 will provide a cubic phase of discontinuous spherical domains of the low volume block arranged in a continuous domain of the higher volume block. As the volume ratio reduces to 70:30, a cylindrical phase will be formed with the discontinuous domains being cylinders of the lower volume block. At a 50:50 ratio, a lamellar phase is formed. With a ratio of 30:70 an inverted cylindrical phase may be formed and at a ratio of 20:80, an inverted cubic phase may be formed.
Suitable BCPs for use as a self-assemblable polymer include, but are not limited to, poly(styrene-b-methylmethacrylate), poly(styrene-b-2-vinylpyridone), poly(styrene-b-butadiene), poly(styrene-b-ferrocenyldimethylsilane), poly(styrene-b-ethyleneoxide), poly(ethyleneoxide-b-isoprene). The symbol “b” signifies “block” Although these are di-block copolymer examples, it will be apparent that self-assembly may also employ a tri-block, tetra-block or other multi-block copolymer.
One method used to guide or direct self-assembly of a polymer (such as a BCP) onto a substrate surface is known as graphoepitaxy. This method involves the self-organization of a BCP guided by topological pre-patterning on the substrate using one or more features constructed of resist (or one or more features transferred from resist onto a substrate surface, or one or more features transferred onto a film stack deposited on the substrate surface). The pre-patterning is used to form an enclosure or “recess” comprising a substrate base and a sidewall, e.g., a pair of opposing side-walls, of resist (or a side-wall formed in a film or a side-wall formed in the substrate).
Typically, the height of a feature of a graphoepitaxy template is of the order of the thickness of the BCP layer to be ordered, so may be, for instance, from about 20 nm to about 150 nm.
A lamellar self-assembled BCP can form a parallel linear pattern of lithography features with adjacent lines of the different polymer block domains in the recesses. For instance if the BCP is a di-block copolymer with A and B blocks within the polymer chain, the BCP may self-assemble into an ordered layer in each recess, the layer comprising regularly spaced first domains of A blocks, alternating with second domains of B blocks.
Similarly, a cylindrical self-assembled BCP can form an ordered pattern of lithography features comprising cylindrical discontinuous first domains surrounded by a second continuous domain. For instance, if the BCP is a di-block copolymer with A and B blocks within the polymer chain, the A block may assemble into a cylindrical discontinuous domain within a circular recess and surrounded by a continuous domain of B block. Alternatively, the A block may assemble into cylindrical discontinuous domains regularly spaced across a linear recess and surrounded by a continuous domain of B block.
Graphoepitaxy may be used, therefore, to guide the self-organization of lamellar or cylindrical phases such that the BCP pattern subdivides the spacing of the side wall(s) of a recess into domains of discrete copolymer patterns.
In a process to implement the use of BCP self-assembly in nanofabrication, a substrate may be modified with a neutral orientation control layer, as part of the graphoepitaxy template, to induce the preferred orientation of the self-assembly pattern in relation to the substrate. For some BCPs used in self-assemblable polymer layers, there may be a preferential interaction between one of the blocks and the substrate surface that may result in orientation. For instance, for a polystyrene(PS)-b-PMMA BCP, the PMMA block will preferentially wet (i.e. have a high chemical affinity with) an oxide surface and this may be used to induce the self-assembled pattern to lie oriented substantially parallel to the plane of the surface. Substantially normal orientation may be induced, for instance, by depositing a neutral orientation layer onto the surface rendering the substrate surface neutral to both blocks, in other words the neutral orientation layer has a similar chemical affinity for each block, such that both blocks wet the neutral orientation layer at the surface in a similar manner. By “normal orientation” it is meant that the domains of each block will be positioned side-by-side at the substrate surface, with the interfacial regions between adjacent domains of different blocks lying substantially perpendicular to the plane of the surface.
In a graphoepitaxy template for aligning a di-block copolymer having A and B blocks, where A is hydrophilic and B is hydrophobic in nature, the graphoepitaxy pattern may comprise hydrophobic resist side-wall features, with a neutral orientation base between the hydrophobic resist features. The B domain may preferentially assemble alongside the hydrophobic resist features, with several alternating domains of A and B blocks aligned over the neutral orientation region between the pinning resist features of the graphoepitaxy template.
A neutral orientation layer may, for instance, be created by use of random copolymer brushes which are covalently linked to the substrate by reaction of a hydroxyl terminal group, or some other reactive end group, to oxide at the substrate surface. In other arrangements for neutral orientation layer formation, a crosslinkable random copolymer or an appropriate silane (i.e. molecules with a substituted reactive silane, such as a (tri)chlorosilane or (tri)methoxysilane, also known as silyl, end group) may be used to render a surface neutral by acting as an intermediate layer between the substrate surface and the layer of self-assemblable polymer. Such a silane based neutral orientation layer will typically be present as a monolayer whereas a crosslinkable polymer is typically not present as a monolayer and may have a layer thickness of typically less than or equal to about 40 nm, or less than or equal to about 20 nm.
A thin layer of self-assemblable BCP may be deposited onto a substrate having a graphoepitaxy template as set out above. A suitable method for deposition of the self-assemblable polymer is spin-coating, as this process is capable of providing a well-defined, uniform, thin layer of self-assemblable polymer. A suitable layer thickness for a deposited self-assemblable polymer film is approximately 10 nm to 150 nm.
Following deposition of the BCP film, the film may still be disordered or only partially ordered and one or more additional steps may be needed to promote and/or complete self-assembly. For instance, the self-assemblable polymer may be deposited as a solution in a solvent, with solvent removal, for instance by evaporation, prior to self-assembly.
Self-assembly of a BCP is a process where the assembly of many small components (the BCP) results in the formation of a larger more complex structure (the nanometer sized features in the self-assembled pattern, referred to as domains in this specification). Defects arise naturally from the physics controlling the self-assembly of the polymer. Self-assembly is driven by the differences in interactions (i.e. differences in mutual chemical affinity) between A/A, B/B and A/B (or B/A) block pairs of an A-B BCP, with the driving force for phase separation described by Flory-Huggins theory for the system under consideration. The use of graphoepitaxy may greatly reduce defect formation. The Flory-Huggins interaction parameter (chi value), and the degree of polymerization of the BCP blocks (N value) are parameters of the BCP which affect the phase separation, and the dimensions with which self-assembly of a particular BCP occurs.
For a polymer which undergoes self-assembly, the self-assemblable polymer will exhibit an order-disorder temperature To/d. To/d may be measured by any suitable technique for assessing the ordered/disordered state of the polymer, such as differential scanning calorimetry (DSC). If layer formation takes place below this temperature, the molecules will be driven to self-assemble. Above the temperature To/d, a disordered layer will be formed with the entropy contribution from disordered A/B domains outweighing the enthalpy contribution arising from favorable interactions between neighboring A-A and B-B block pairs in the layer. The self-assemblable polymer may also exhibit a glass transition temperature Tg below which the polymer is effectively immobilized and above which the copolymer molecules may still reorient within a layer relative to neighboring copolymer molecules. The glass transition temperature is suitably measured by differential scanning calorimetry (DSC).
Defects formed during ordering as set out above may be partly removed by annealing. A defect such as a disclination (which is a line defect in which rotational symmetry is violated, e.g. where there is a defect in the orientation of a director) may be annihilated by pairing with other another defect or disclination of opposite sign. Chain mobility of the self-assemblable polymer may be a factor for determining defect migration and annihilation and so annealing may be carried out at a temperature where chain mobility is high but the self-assembled ordered pattern is not lost. This implies temperatures up to a few ° C. above or below the order/disorder temperature To/d for the polymer.
Ordering and defect annihilation may be combined into a single annealing process or a plurality of processes may be used in order to provide a layer of self-assembled polymer such as BCP, having an ordered pattern of domains of differing chemical type (of domains of different block types).
In order to transfer a pattern, such as a device architecture or topology, from the self-assembled polymer layer into the substrate upon which the self-assembled polymer is deposited, typically a first domain type will be removed by so-called breakthrough etching to provide a pattern of a second domain type on the surface of the substrate with the substrate laid bare between the features of the second domain type. A pattern having parallel cylindrical phase domains can be etched using a dry etching or reactive ion etching technique. A pattern having lamellar phase domains can utilize a wet etching technique in addition to or as an alternative to those suitable for the etching of parallel cylindrical phase domains.
Following the breakthrough etching, the pattern may be transferred by so-called transfer etching using an etching means which is resisted by the second domain type and so forms recesses in the substrate surface where the surface has been laid bare.
Spacing between lithography features is known as pitch—defined as the width of one repeat unit of the lithography feature (i.e. feature width plus inter-feature spacing). A self-assembly process using a BCP can be used to produce lithography features with particularly low pitch, typically less than 30-50 nm.